The invention relates to a cryostat arrangement comprising a vacuum vessel in which a superconducting magnet coil system to be cooled is arranged, a cryocooler that actively cools the cryostat arrangement. The cryocooler comprises a coolant circuit with a compressor and a cold head, which may have a single-stage or dual-stage cooling arm that is in thermal contact with the superconducting magnet coil system. The cryostat further comprises a volumetric vessel containing a cryogenic fluid, such as helium. The volumetric vessel is arranged such that it is thermally conductively connected to the superconducting magnet coil system and/or to parts of the cryostat arrangement enabling ambient heat to flow to the superconducting magnet coil system.
Nuclear magnetic resonance apparatuses, in particular for NMR spectrometry or NMR tomography, require strong magnetic fields, which are often generated by superconducting magnet coils. The superconducting magnet coils are operated at a cryogenic temperature. The magnet coils are typically arranged in a “non-dry” cryogenic vessel of a cryostat, which is filled with a cryogenic liquid such as liquid helium. In order to maintain the operating temperature on a long-term basis and at the same time minimize the consumption of cryogenic liquids, the cold finger of a cold head projects into the cryogenic vessel to absorb heat. The cryogenic vessel is surrounded by a vacuum vessel for thermal insulation. However, many users prefer “cryogen-free” magnets, which largely forgo the use of cryogenic liquids (e.g. liquid helium and/or liquid nitrogen) and maintain the operating temperature exclusively by means of “cryocoolers”, i.e. in a “dry” manner. Pulse tube, Stirling or Gifford-McMahon coolers are typically used in this case.
A cryogen-free magnet system of this kind typically have a very short time to quench (TTQ). If the cryocooler malfunctions (e.g., as a result of a power outage, an interruption to the cooling water supply, or a mechanical defect in the compressor or cold head), the magnet system very quickly heats up beyond the allowable operating temperature, the superconductivity breaks down, and the magnet system quenches. The magnet system then cannot be used for a long time, since it must be cooled back down and recharged.
For this purpose, separate systems may be provided for automatically filling the reservoir after a malfunction of the active cryocooler. Currently, a gas cylinder, normally filled with helium, may be used. However, repeated cryocooler malfunctions may leave the gas cylinder empty, requiring the gas cylinder to be replaced.
Another option is to connect the reservoir to a storage volume, typically at room temperature, into which the cryogen can flow out. The storage volume typically has to be very large in order to prevent an impermissibly sharp increase in pressure.
Another option is to design the helium reservoir and the storage volume (if provided) to be hermetically sealed and mechanically very stable, such that they withstand the extremely high pressures that build up when helium is vaporized and heated in a small, closed space. A reservoir of this kind is compact, but is very heavy and is also relatively expensive. It is particularly disadvantageous that the pressure is at its highest in warm conditions and drops during operation, i.e. the reservoir is oversized for the pressures prevailing in normal operation.
According to the European patent document EP 0 937 953 A1, the TTQ can be prolonged by storing small amounts of a cryogen in a reservoir provided for this purpose. The device described is disadvantageous, however, in that the storage volume has to be very large (e.g., 1200 liters in the practical example of the patent document).
In the cryostat arrangement according to document German patent reference DE 199 14 778 B4, the TTQ is prolonged by storing small amounts of a cryogen in a reservoir provided for this purpose. Here too, the reservoir is connected to a separate storage volume that has to be very large.
In the German patent reference DE 10 2014 218 773 A1, a completely different approach is taken to prolong the TTQ. Instead of increasing the thermal capacity and the available thermal capacity, the thermal coupling between the coil and the cryocooler is reduced if the active cryocooler malfunctions.
The U.S. patent reference U.S. Pat. No. 7,263,839 B2 describes providing some liquid helium in a cryogen-free system in order to prolong the TTQ. However, the gas circuit of the active cooler is not used to prolong the TTQ here, and, in one of the embodiments described, this requires a separate compressor and therefore increases the complexity of the system.
In the U.S. patent reference U.S. Pat. No. 5,410,286, a small amount of liquid helium is used in an otherwise “dry” system. However, the gas circuit of the active cryocooler is not used to prolong the TTQ. In the embodiments described, the cryogen escapes into the atmosphere instead.
In the international patent reference WO-2016/038093 A1, the TTQ is prolonged by providing a reservoir containing a cryogen. Here, however, the reservoir is designed to be completely hermetically sealed. When the reservoir is heated, an extremely high pressure builds up in the interior. The reservoir therefore has to be accordingly sturdy, and therefore large, heavy, and expensive.